The title of James Ellroy's latest novel is taken from AE Housman's poem "Reveille", a very English meditation on life's brevity. Four lines from the poem also provide the book's epigraph. Discovering that Ellroy digs Housman is one of the few surprising things about Blood's a Rover, the concluding part of the novelist's Underworld USA trilogy. It's as if Metallica had decided to call an album after a work by Vaughan Williams.

In Ellroy's fiction, blood is more often a river. Though he has long since transcended the formal constraints of the traditional crime novel, he remains wedded to its hard-boiled tone, a tone that he, more than anyone else, has contemporised. Here, as with the preceding novels that make up the trilogy, American Tabloid (1995) and The Cold Six Thousand (2001), the labyrinthine plot demands that the reader pays total attention throughout while simultaneously being beaten into submission by sentences that are often nasty and brutish and always short. It's quite a style and one marvels once again at the obsessively brilliant brain behind it. But, boy, is it exhausting.

Itbegins characteristically with a vivid flashback: a cinematically violent description of a highway robbery, LA-style, in 1964. "The milk truck driver pulled a silencered piece and shot the nearest guard in the face. The noise was a thud. The guard's face exploded. The two other guards fumble-grabbed at their holsters. The masked men shot them in the back. They buckled and pitched forward. The masked men shot them in the head point-blank. The thuds and skull crack muffle-echoed. It's 7.19am. It's still quiet."

There is much to admire here, not least the Joycean ingenuity of "fumble-grabbed" and "muffle-echoed" and the deadpan black humour of the pay-off line. At around 600 pages, though, Blood's a Rover, like its equally dense precursors, is an awful lot of short sentences. Even a third of the way in I was longing for a respite from the machine gun prose, for just one Rothian passage, a sentence that would snake on and on luxuriantly into a long paragraph. Some hope.

Since American Tabloid propelled him out of the epically adventurous but still recognisably Chandleresque territory of his brilliant LA Quartet – The Black Dahlia, The Big Nowhere, L.A. Confidential and White Jazz – Ellroy has adhered to this staccato but epic style. His novels also tend toward the metafictional in making use of real events, multiple, often unreliable, narrative points of view and "document inserts" – FBI and police records, journal entries, interview transcripts. His aim is to retell in a new, illuminating way the more turbulent episodes in late 20th-century American history from the point of view of those involved at both the highest levels of political power and the lowest levels of criminal activity. His subtext, though, is an old one: it was ever thus.

Here, as before, it is the assassinations of JFK, Robert Kennedy and Martin Luther King that cast a shadow over the action, while the cover-ups and conspiracies that attend the Nixon era provide the murky political and cultural landscape that Ellroy navigates in his inimitably obsessive fashion. Thankfully, on the conspiracy front, he is more Don DeLillo than Oliver Stone – it was DeLillo's Libra that influenced him most when plotting the trilogy.

As always, what entrances and appals is the extremity of Ellroy's vision. Blood's a Rover is not a book for the politically correct reader, nor the overly sensitive one. Corruption, in Ellroy's world, is always total and extreme. Only Wayne Tedrow Junior survives from The Cold Six Thousand, but now he's no longer an LAPD man, but a drug runner. He has also slept with his stepmother, Janice, who is now dying of cancer. He plies her with heroin for the pain. They have already killed his father, her ex-husband. Or, as Ellroy puts it, by way of bringing the reader up to speed: "Wayne decided to murder his father. Wayne decided that Janice should beat him dead with a golf club."

In Ellroy's world, every plot, however sub, congeals; every character, however amoral, is capable of surprising us – and himself – with some new sin. So it is with the two other main characters, Dwight Holly and Don "Crutch" Crutchfield, the one an FBI agent who is a graduate of Yale Law School and whose daddy was big in the Ku Klux Klan, the other a 23-year-old loser from a bad family who has landed a job as a wheelman. (For the uninitiated, a wheelman does the menial work for "skank private eyes and divorce lawyers", which makes them, as Ellroy succinctly puts it, "low-rent and indigenously fucked-up".)

Against a backdrop in which Nixon comes to power and America's cities explode into violent protest, Wayne, Dwight and Crutch chase their own tainted dreams of power and/or revenge. Wayne works for Howard Hughes, who is holed up in Vegas, but has both the Mob and the Feds on his tail. Dwight is trying to infiltrate a radical black nationalist organisation called, with Ellroy's usual liberal-baiting relish, the Black Tribe Alliance and the Mau Mau Liberation Front. Only Crutch is keeping it real by conforming to the conventions of the crime novel. He is chasing a femme fatale – Gretchen Farr aka Celia Reyes – who he hopes will lead him to the even more mysterious Joan Klein. If, by this stage, you are still looking for answers, Joan may have them. Me, I was still trying to figure out the questions.

Throughout, there are the usual suspects – bent cops, even more bent politicians, conmen, molls and a cast of venal and corrupt men, some of whom – against all the odds – possess just a sliver of conscience. They remain, as in all Ellroy's fiction, in the minority. It is Crutch, the most shady and sordid of the triumvirate of central characters, who redeems himself, emerging from the wreckage of the time to testify to its cataclysmic import.

For all the vivid pencil sketches, Ellroy is not big on characterisation and the density of the plot may leave all but the utterly committed utterly confused. Then again, The Cold Six Thousand made the American bestseller list, which suggests that Ellroy's late style, once surrendered to, may prove strangely addictive. "You will read with some reluctance and capitulate in the end," writes the unlikely narrator, the chief witness, in his preamble. "The following pages will force you to succumb." That about nails it.